1. Field of the Invention
The present invention relates to a non-linearly approximation circuit and a method thereof for video signals with a look-up table, in particular to a gamma correction circuit and a method thereof being applicable to video signal processing apparatus of a television (TV), a TV camera and so on.
2. Description of the Prior Art
Generally, a gamma correction circuit of an analogue type is described in, for example, U.S. Pat. No. 3,970,774, entitled "Electronic Signal Mixer" and U.S. Pat. No. 4,038,685, entitled "Apparatus for Automatic Gamma Control of Television Color Signals". In U.S. Pat. No. 3,970,774, a first output obtained by additively or differentially combining first and second signals, is combined with a product output which is also obtained by additively or differentially combining the first and second signals and further multiplying the combined signal by a coefficient, so as to provide an output signal having a desired ratio of the first and second signals in accordance with the coefficient. In U.S. Pat. No. 4,038,685, a gamma correction circuit is controlled in accordance with the comparison results obtained by comparing R, G and B color television signals with first and second levels, thereby automatically and continuously correcting the gamma characteristics.
In order to deal with the latest digital processing of a video signal, another method of gamma correction is known wherein the values of output signals, g(x, .gamma.)=x.gamma. in which a gamma correction with a gamma correction coefficient .gamma. is made, are calculated in advance and stored in a so-called look-up table in the form of a pair of the input signal x and the output signal x.gamma.. The present invention relates to a gamma correction circuit having such a look-up table.
In general, it is preferable that the gamma correction coefficient .gamma. is variable and finely adjustable over the range from 0.35 to 0.55, but in accordance with the above-mentioned look-up table method, an amount of the data for the variable coefficients greatly increases. For example, when the number of quantized bits is eight bits, 256 bytes are necessary for a certain kind of the gamma coefficient. When it is required to vary the gamma coefficient with ten levels, the data becomes 2560 bytes.
Namely, in order to perform the gamma correction finely, the necessary memory capacity of the look-up table is increased.